The present invention relates to a method of applying a coating fluid to the surface of a support and more particularly to a method of applying a solution comprising photo-sensitive material, magnetic material, etc., dissolved in a solvent with an extrusion type coating head of non-pressurizing type.
One of conventional methods of manufacturing various types of photographic film, photographic paper, and magnetic recording media is to extrude a coating fluid comprising photo-sensitive material, magnetic material, treatments, etc., dissolved in a solvent such as an organic solvent from a slit of a coating head to the surface of a running long support for forming coats made of the materials on the support surface.
Since generally a multitude of coats are applied, the following two application methods are adopted: Wet on dry method by which application and drying steps are repeated for each coat to form multiple coats and wet on wet method by which as many coating heads as the number of coats are disposed along the move direction of a support or a coating head formed with as many slits as the number of coats is used to extrude coating fluid from the slits to the application face of the support at the same time and laminate the coats on each other in a wet condition. Of the two methods, the latter is useful because multiple coats can be formed consecutively.
The wet on wet application method is described in Japanese Patent Laid-Open Nos.Sho 61-139929 and 58-205561, for example. The application method described here is to press a coating head called an extruder (extrusion type coating head) against the application face of a running long support and extrude a coating fluid from a slit of the extruder for forming a coat on the support.
The wet on wet application method using the extruder mentioned above is a so-called pressurizing type application method of applying a coating fluid with the back edge of the extruder pressed against the support. However, since the coating fluid is pressed against the support on the back edge face in the pressurizing type application, the coating fluid may seep out in the width direction of the support, causing an uneven coating face to be formed.
To solve such a problem in the pressurizing type application and further enable high speed thin coat application, a non-pressurizing type extruder as described in Japanese Patent Laid-Open No.Sho 63-20069 has been developed, for example. As shown in the FIGURE, the non-pressurizing type extruder 1 comprises a front edge 3 positioned in the upstream with respect to the running direction of a support 2 (arrow A direction) and a back edge 4 positioned in the downstream and is provided with a level difference so that the tip 41 of the back edge 4 recedes from the tip 31 of the front edge 3 with respect to the support 2 (so that it is positioned upward in the FIGURE).
A coating fluid 5 is extruded toward the support 2 from a slit 6 defined by the front edge 3 and the back edge 4. At the time, the back edge 4 is out of contact with the support, so that the coating fluid 5 is applied without being subjected to smoothing action by the back edge face 40.
However, since the coating fluid 5 is out of contact with the support 2 in press condition in the application method using the non-pressurizing type extruder, which will be hereinafter called simply the non-pressurizing type application, air mixed from the front edge accompanying the application is not excluded when a coat is formed. Stripes will occur on the coating face or the coat thickness may change depending on the coating face condition of the support 2. Thus, normally a precoat solution 7 is used for a liquid seal.
For the liquid seal, the precoat solution 7 is previously applied to the application face of the support 2 in the upstream of the extruder 1 and when the support 2 arrives at the extruder 1, excessive precoat solution 7 is scraped out by the front edge 3 and the gap between the front edge face 30 and the support 2 is sealed with the precoat solution 7, thereby preventing air from being mixed into the coating fluid 5, flattening out the application face of the support 2 regardless of the surface condition thereof, and preventing excessive coating fluid pressurization from placing behavior of the coating fluid 5 out of order unexpectedly for improving high speed thin coat application performance. Further, dust adhering to the support 2 is taken into the precoat solution 7 and when arriving at the extruder 1, can be removed together with the excessive precoat solution 7 by the front edge 3. Foreign material is not trapped in the tip 41 of the back edge 4.
The coating fluid 5 is extruded from the slit 6 and laminated on a precoat 70 whose thickness is defined in passage through the front edge 3 for forming a coat 50. The effect of the precoat solution 7 as described above makes the coat 50 a good thin film free of stripes or thickness change. Therefore, the non-pressurizing type application method is extremely suitable particularly for high speed thin coat application.
By the way, the precoat solution 7 generally is a single organic solvent or a solution consisting essentially of an organic solvent containing a binder, etc., in order to improve adherence of the support 2 and the coating fluid 5 to each other. The precoat solution 7, which is removed from the support 2 after the coating fluid 5 is applied, preferably has higher volatility. Further, it is a component not contained in the final product and preferably is as inexpensive as possible.
In contrast, the coating fluid 5 is a solution comprising various substances including magnetic material, photo-sensitive material, etc., dissolved in an organic solvent. Thus, for the solvent contained in the coating fluid 5, a solvent having low volatility may have to be mixed or an expensive solvent may have to be used to uniformly dissolve or disperse all component substances contained in the coating fluid 5, for example, magnetic material, a binder, an antistatic agent, a lubricant additive, etc., in a magnetic coating fluid.
To manufacture multiitem products, often the type of coating fluid 5 is changed for application by using common equipment for reasons of facilities. In this case, often the coating fluid 5 only is changed and the same precoat solution 7 is used.
Thus, in many cases, the solvent contained in the precoat solution 7 differs from the solvent contained in the coating fluid. A chemical or physical affinity between the precoat solution 7 and coating fluid 5 may be lacked, causing a so-called "runaway phenomenon" to occur due to differences between the solvents contained in both the solution and fluid in polarity, density, surface tension, mutual solubility, etc., depending on a combination of the precoat solution and coating fluid.
Resultantly, adherence of the coating fluid 5 will lower and the coat film thickness will become uneven or the coat may flake off. Particularly, since the coating fluid 5 is applied via the precoat solution 7 inevitably in the non-pressurizing type application, solvent combinations must be sufficiently considered.
Proposals related to the "runaway phenomenon" were presented in Japanese Patent Laid-Open Nos.Hei 2-105331 and 3-119518. However, the contents disclosed here are applied to multiple magnetic coats and are related to the pressurizing type application of pressuring a coating fluid on the edge face as described above; they differ from the non-pressurizing application type to which the invention is applied in premise technique. An intensive research of the inventor and associates shows that the techniques disclosed in Hei 2-105331 and 3-119518 cannot be applied intact.